It has long been known that ultrasonics may be used to provide an accurate indication of various physical properties of a wide variety of objects including the human eye, tubing and pneumatic tires without destruction of the object.
In one exemplary technique, known as quantitative section analysis, dimensional characteristics of an object are found by generating an image of a section of the object along a designated inspection line using an ultrasonic beam. If such an analysis is to be most meaningful, the location of the ultrasonic beam must be accurately located relative to the geometry of the object under test.
A spectrum of techniques have been used in an effort to identify the location of the invisible ultrasonic beam, generally without great accuracy and precision, and with substantial expense and complexity.
It has been recognized that, at least when examining the human eye, visible light is a desirable medium to aim and thereby locate an ultrasonic beam. In U.S. Pat. No. 3,371,660, ultrasonic waves utilized for eye examinations are introduced to the eye coaxially with light directed through a conventional ophthalmoscope. By viewing the eye and a light marker thereon through the ophthalmoscope, the operator can identify the location of ultrasonic waves incident on the eye exterior. However, because the focal points of both the ultrasonic waves and light beam are coincident, any attempt to focus the ultrasonic beam inside the eye will result in a loss of aiming beam sharpness. In this case the operator cannot accurately determine the location inside the eye of the ultrasonic beam.